Concepts of salts

Salt is an ionic compound that has a cation other than H+ and an anion other than OH– and is obtained along with water in the neutralization reaction between acids and bases.

Types of Salts

1. Acidic salt – This salt is formed by partial neutralization of a diprotic or a polyprotic acid and is known as an acidic salt. These salts have ionizable H+ ions along with another cation. 

For eg:- NaHSO4­, KH2PO4, etc. are acidic salts.

H2SO4  + NaOH → NaHSO4 + H2O

2. Basic or Alkali Salt – The salt formed due to the partial neutralization of a strong base by a weak acid is known as a basic salt.  When hydrolysis of basic salt takes place, the conjugate base of the weak acid is formed in the solution which makes the overall solution basic.

For eg:-  Basic copper chloride (Cu(OH)Cl), Basic magnesium chloride (Mg(OH)Cl), etc.

3. Double salt – The salts that contain more than one cation or anion are known as double salt. They are formed by the combination of two different salts when they crystallize in the same ionic lattice.

For eg:- Carnallite is a salt of potassium chloride and magnesium chloride ( KCl.MgCl2.6H20)

Potash alum is also a double salt that is widely used in our daily lives (K2SO4.Al2(SO4)3.24H2O).

4. Mixed Salts – Mixed salts are salts containing either two different cations with a common anion or two different anions with a common cation.

For e.g. :- CaOCl2 (Bleaching powder), sodium potassium carbonate( NaKCO3) etc.

Difference between a double salt and a complex

Complexes and double salts are formed by the combination of two or more stable compounds in a fixed proportion.  However, they differ in the fact that double salts dissolve into simple ions completely when dissolved in water. However complex salts like Potassium ferrocyanide do not dissociate into Fe3+ and CN- ions.

Some special properties of salts

Deliquescence

Deliquescence is the phenomenon through which a compound absorbs moisture from the surrounding until it dissolves in it to form a solution. Certain substances have the property to absorb moisture when exposed to the surroundings at any temperature, they initially become wet, lose their crystalline form and finally dissolve in water to form a saturated solution. 

For example-  MgCl2, CaCl2, FeCl3, LiCl,  Cu(NO3)2, NaNO3, LiNO3 etc are deliquescent substances. 

Efflorescence

Certain hydrated crystalline salts when exposed to the air for a long period lose their water of crystallization molecules either partially or completely and become anhydrous. This phenomenon occurs when the aqueous vapour pressure of the compound in its hydrate form is greater compared to the partial pressure of the water vapour in the air. 

For example:- Glauber’s Salt (Na2SO4.10H2O), Green vitriol (FeSO4.7H2O), Blue Vitriol (CuSO4.5H2O), washing soda (Na2CO3.10H2O) loses 9 water molecules first and then become completely anhydrous.

Hygroscopy

Certain substances absorb moisture from the surroundings if exposed for a significant duration of time but do not dissolve in it. The property is hygroscopy and the substances are called hygroscopic.

For eg:- Zinc chloride, Sodium chloride, etc.

Drying Agents

Some commonly used chemicals as drying agents are calcium chloride (CaCl2), calcium sulphate (CaSO4), sodium sulphate (Na2SO4), magnesium sulphate (MgSO4) in their anhydrous form.

Calcium chloride is suitable for drying neutral gases and many halogenated and aromatic carbons.

Granular calcium chloride is also used in drying tubes.

Some Important Salts of sodium and calcium. 

Sodium hydroxide

Chemical formula – NaOH(Also known as  caustic soda)

Preparation (Chlor-alkali process):

1: Electrolysis of brine (solution of common salt, NaCl) is carried out.

Chlorine is released in the anode while hydrogen is released at the cathode.

NaOH is left behind as residue in the solution.

2: Commercially, it is prepared by the electrolysis of NaCl in the Castner-Kellner cell.

Bleaching powder

Chemical formula – Ca(OCl)Cl or CaOCl2

Preparation 

– Ca(OH)2(aq) + Cl2(g) → CaOCl2(aq) + H2O(l)

Baking soda

Chemical name – Sodium hydrogen carbonate

Chemical formula – NaHCO3

Preparation (Solvay process):

a. Limestone is heated: CaCO3→CaO+CO2

b. CO2 is passed through a concentrated solution of sodium chloride and ammonia:

NaCl(aq)+NH3(g)+CO2(g)+H2O(l)→NaHCO3(aq)+NH4Cl(aq)

Uses:

1. Used in the manufacture of glass, soap, borax, and caustic soda.

2. Used in the paper, textile, and paints industry.

3. It is an important laboratory reagent both in qualitative and quantitative analysis.

Washing soda

Chemical name – Sodium carbonate

Chemical formula – Na2CO3.10H2O

Preparation (Solvay process) – 

In this process, the advantage is taken of the low solubility of sodium hydrogen carbonate whereby it gets precipitated in the reaction of NaCl with NH4HCO3. NH4NHCO3 is prepared by passing CO2 to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogen carbonate is formed. NaHCO3 crystal separates from the solution. This when heated gives sodium chloride.

The Solvay process can be extended to the manufacture of potassium carbonate because potassium hydrogen carbonate is too soluble to be precipitated by the addition of NH4HCO3 to a saturated solution of KCl.

Uses

1. In the glass, soap, and paper industries

2. Softening of water

3. Domestic cleaner

Calcium Oxide/ Quicklime(CaO)

Preparation:

It is prepared on a commercial scale by heating limestone(CaCO3) in a rotary kiln at 1070-1270K.

Uses:

In the purification of sugar and manufacture of dyes and stuff.

In the manufacture of sodium carbonate from caustic soda.

Conclusion:

Salt has wide applications in the industries. From our table salt to potash alum which is used to purify water or as an antiseptic, we use them knowingly or unknowingly. Drying/Desiccating agents like silica gel are always present in shoes, electronics to keep them dry. They also act as electrolytes, thus having huge importance in our lives.